The present invention relates generally to the preparation of mixtures and solutions, and particularly to a system which is capable of repeatably dispensing a very precise weight percent concentration of a fluid to a selected material.
Analytical grade solutions are typically prepared by skilled technicians using a manually implemented procedure. This procedure generally begins by adding the sample material or analytical standard material to a container until the weight of the material is within a prestated acceptable weight range. Then, the material is diluted with a solvent until a constant volume is reached. As a result, the solution concentration will vary within the weight variation of the sample/standard material, and there may also be some variability due to the volume of solvent added. While the incorporation of an internal standard in the solvent may adjust for the volume uncertainty, additional variability may nonetheless exist due to variations in the solvent or internal standard solution weight with temperature.
This procedure may be automated by a robot system, such as that available from Zymark Corporation, Zymark Center, Hopkinton, Mass. 01748. This type of robotic system is capable of weighing the amount of liquid solvent added as part of the sample preparation. However, no control of the ratio of the sample to solvent weight is provided. Additionally, the manner of adding the sample slowly (e.g., by gently tapping the container) is relatively slow and may expose the sample to the atmosphere for a lengthy time. Such exposure could lead to possible moisture or decomposition concerns with some sample materials (e.g., the loss of volatile components). Additionally, the presence of vibrations (e.g., from tapping) is known to cause particle size discrimination which could lead to non-representative sampling of non-homogeneous materials.
Another approach to preparing analytical solutions is represented by the Stanprep dispenser manufactured by Spiral Systems, Inc., Cincinnati, Ohio. A description of this type of dispenser is presented in Campbell et. al. U.S. Pat. No. 4,469,146, issued on Sep. 4, 1984, and entitled "Apparatus And Process For Preparing Quantitative Chemical Solutions". This dispenser apparatus utilizes a table of solution densities to estimate the weight amount of liquid solvent to add. This amount of solvent is then added and weighed. If the weight is under that estimated, then a correction is made by adding more solvent until the initially calculated solvent plus solute weight is reached. However, if the weight is over that initially estimated, then the apparatus responds with an error indicator. Additionally, since densities are not constant, there may be some concentration variability with this apparatus due to density shifts with temperature. This apparatus also requires extensive density tables to be created, either by estimation or by empirical measurement.
Accordingly, it is a principal objective of the present invention to provide a rapid and automatic method and system for dispensing a series of fluid volumes to repeatably create mixtures or solutions having a very precise weight percent concentration.
It is a more specific objective of the present invention to provide a net weight dispensing method and system which controls the final weight percent concentration, rather than attempting to hold volume or weight to an arbitrary tolerance.
It is an additional objective of the present invention to provide a net weight dispensing method and system which may be operated with only a minimal amount of training.
It is a further objective of the present invention to provide a net weight dispensing method and system which is not sensitive to density variations when solutions are prepared at different times or under different conditions.
It is another objective of the present invention to provide a net weight dispensing method and system which is capable of self-calibration in connection with each dispensing operation.